Various commercially available after-market devices have been designed to intercept radio frequency (RF) transmissions at an input of an RF receiver and inject a different transmission signal into the RF receiver. In general, these devices use existing RF receivers, e.g. frequency modulation (FM) receivers, to implement additional audio functions, e.g., mobile phones, satellite digital audio radio systems (SDARS) and compact disc (CD) players. Many of these devices detect a local oscillator (LO) frequency of an FM receiver to determine a current channel of the receiver (tuner). In general, such after-market devices have been inserted between the receiver and its associated antenna.
These devices have typically relied on the presumption that FM receivers use the same tuner design or the same intermediate frequencies. Such devices work relatively well with FM receivers that have sufficient LO leakage such that the intermediate frequencies can be detected. However, there are many FM receivers that suppress LO leakage and/or use different tuner architectures and, in these situations, the currently available after-market devices do not function adequately. In receivers that suppress LO leakage, an operator of the receiver has been required to tune to a fixed frequency of operation for the after-market device. Unfortunately, this eliminates any ability for an added device to interrupt the current receiver channel, unless a user is on that channel.
What is needed is a device that, when placed between a radio frequency (RF) receiver and its associated antenna, determines a current channel of the receiver.